The present invention relates to a tunable wavelength filter, and more particularly, it relates to a tunable wavelength filter on use of a liquid crystal etalon, which is independent of polarization.
The tunable wavelength filter is used for electing a light of desired wavelength in optical pulses of a number of wavelengths selectively in order to transmit a great capacity of informations in the field of optical communication using an optical fiber.
As the conventional tunable wavelength filter, and more particularly, as the tunable wavelength filter for multiple-communication separating wavelength, there are known a mechanically controlled grating and a Fabry-Perot etalon consisting of two adjustable mirrors mounted in parallel with each other. However, these filters contains such problems as largeness in size, requirement of high driving power, low reliability, high cost and so on.
In order to solve such problems, there was proposed a liquid crystal etalon type of tunable wavelength filter in which a liquid crystal layer is interposed between two sheets of glass substrates each of which has a surface opposing to each other and having a transparent electrode, a reflection layer and an alignment layer coated thereon. This liquid crystal etalon type of tunable wavelength filter has a potential for realizing the filter of down-sized, low electrical power and low cost.
Shown in FIG. 1 is the conventional liquid crystal etalon type of tunable wavelength filter which is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 4-220618. In the figure, reference numeral 1 designates glass substrates, 2 anti-reflection coatings, 3 transparent electrodes, 4 dielectric mirrors as reflection layers, 5 alignment layers, 6 a liquid crystal, 7 a spacer and 8 a lead wire.
In the example, the liquid crystal 6 is interposed between the glass substrates 1 inside which the transparent electrodes 3, the dielectric mirrors 4 and the alignment layers 5 are laminated in sequence and oriented homogeneously so as to be in counter-parallel with each other between the glass substrates 1 in which crystal molecules oppose to each other.
FIG. 2A is a diagram showing a change of peak wavelength passing through the tunable wavelength filter of the above publication in case of changing a voltage applied on the filter, namely, a dependency of peak wavelength on voltage applied thereon. By this diagram, it will be understood that the more voltage is applied on the tunable wavelength filter, the peak-wavelength is shifted from the longer wavelength region to shorter wavelength region.
U.S. Pat. No. 5,068,749 also discloses another tunable wavelength filter which is similar to the above-mentioned filter of JPP No. 4-220618 except that crystal molecules charged between the opposing glass substrates are arranged in accordance with the twist alignment of twist of n.pi./2 radian (n: a positive uneven number). FIG. 2B is a diagram showing a dependency of peak wavelength of the tunable wavelength filter disclosed in the above publication on voltage applied thereon. As will be apparent from the figure, by applying the voltage on the filter, the peak wavelength is shifted from the longer wavelength region to the shorter wavelength region. Note, throughout FIGS. 2A and 2B, curves shown with black and white square marks (.quadrature.;.box-solid.) designate the respective outputs of liquid crystal etalon in response to a variety of incident lights by representing a parameter of translucent wavelength for the voltage applied thereon. As shown in the figure, whatever polarization condition the incident light may take, two kinds of output curves are obtained. Besides, in case that the incident light under a linearly polarization condition impinges in parallel or vertically to the liquid crystal molecules, only single kind of output will be obtained. The reason why the two kinds of curves is because each liquid crystal molecule has a characteristic of index anisotropy that the refractive index of crystal molecules in the longitudinal direction is different from that in the latitudinal direction. Therefore, the incident light is transmitted in the liquid crystal layer at the respective indexes, so that two linearly polarizations are generated. That is, the above-mentioned two linearly polarizations correspond to the curves plotted with the marks (.quadrature.;.box-solid.), respectively. On the contrary, in case that it does not depend on the polarization, only one output will be obtained in any conditions of polarization. In the tunable wavelength filter disclosed in the publication JPP No. 4-220618, however, there is confirmed output lights with two kinds of curves for the incident light as shown in FIG. 2A, which depend on the polarization apparently. Therefore, in order to make the output lights independent of the polarization, it requires a polarization light split, a prism or the like, so that there are raised problems that such an instrument is apt to be large-sized and the cost thereof is increased.
On the other hand, in the filter disclosed in the publication U.S. Pat. No. 5,068,749, as shown in FIG. 2B, two curves are overlapped with each other only in a section of high-voltage, so that it represents a characteristic of independent of the polarization in the same section. However, since the filter represents a feature dependent of the polarization in the section of low-voltage, there is caused a problem that it has a narrow tunable wavelength area with a range of approximately 15 (nm) in the polarization independent area.
Next, we consider the problems which would be caused, in case that the above-mentioned conventional tunable wavelength filter is applied for a spectro-photometer. Note that, the spectro-photometer analyzes an optical light to be inspected and detects a wavelength component thereof and a strength of the detected light at mentioned wavelength. From these points of view, it is desired that the spectro-photometer analyzes the light in a broader range of wavelength and divides the light into individual wavelengths.
From the point of view, in the conventional spectro-photometer, a plurality of band-pass filter, of which translucent wavelength are continuous substantially, are so arranged in array as to form a "grating" thereby to analyze the light by the grating. Respective band-pass filters have a translucent characteristic that respective selective wavelengths are 800 nm, 810 nm, 820 nm . . . , 870 nm in order to pick up a single wavelength only.
The spectroscopic analysis by the spectro-photometer is carried out by either changing the band-pass filters to which the light is radiated, in turn, by means of moving the grating by the mechanical control or emitting the light to be measured against the whole grating.
In the conventional spectro-photometer, however, in order to pick up a light of single wavelength from the light in a wide range of wavelength area, it is necessary to move the grating with high accuracy and to improve the accuracy of the wavelength selectivity of the band-pass filter. Therefore, there are raised problems that the size and cost of spectro-photometer are increased.
Further, even if the above-mentioned tunable wavelength filter is used for the above spectro-photometer, it is impossible to realize the wavelength selectivity characteristic sufficiently because of its dependency of the polarization of light to be measured. In this case, for the purpose of independency of the polarization, since it requires the polarization split and the prism etc. and the tunable wavelength range becomes to be narrowed as mentioned above, there is caused a problem of difficulty of application for the spectro-photometer.